(* Title: HOL/Induct/Sigma_Algebra.thy
Author: Markus Wenzel, TU Muenchen
*)
section \<open>Sigma algebras\<close>
theory Sigma_Algebra
imports Main
begin
text \<open>
This is just a tiny example demonstrating the use of inductive
definitions in classical mathematics. We define the least \<open>\<sigma>\<close>-algebra over a given set of sets.
\<close>
inductive_set \<sigma>_algebra :: "'a set set \<Rightarrow> 'a set set" for A :: "'a set set"
where
basic: "a \<in> \<sigma>_algebra A" if "a \<in> A" for a
| UNIV: "UNIV \<in> \<sigma>_algebra A"
| complement: "- a \<in> \<sigma>_algebra A" if "a \<in> \<sigma>_algebra A" for a
| Union: "(\<Union>i. a i) \<in> \<sigma>_algebra A" if "\<And>i::nat. a i \<in> \<sigma>_algebra A" for a
text \<open>
The following basic facts are consequences of the closure properties
of any \<open>\<sigma>\<close>-algebra, merely using the introduction rules, but
no induction nor cases.
\<close>
theorem sigma_algebra_empty: "{} \<in> \<sigma>_algebra A"
proof -
have "UNIV \<in> \<sigma>_algebra A" by (rule \<sigma>_algebra.UNIV)
then have "-UNIV \<in> \<sigma>_algebra A" by (rule \<sigma>_algebra.complement)
also have "-UNIV = {}" by simp
finally show ?thesis .
qed
theorem sigma_algebra_Inter:
"(\<And>i::nat. a i \<in> \<sigma>_algebra A) \<Longrightarrow> (\<Inter>i. a i) \<in> \<sigma>_algebra A"
proof -
assume "\<And>i::nat. a i \<in> \<sigma>_algebra A"
then have "\<And>i::nat. -(a i) \<in> \<sigma>_algebra A" by (rule \<sigma>_algebra.complement)
then have "(\<Union>i. -(a i)) \<in> \<sigma>_algebra A" by (rule \<sigma>_algebra.Union)
then have "-(\<Union>i. -(a i)) \<in> \<sigma>_algebra A" by (rule \<sigma>_algebra.complement)
also have "-(\<Union>i. -(a i)) = (\<Inter>i. a i)" by simp
finally show ?thesis .
qed
end